Simulation is a powerful tool for characterizing, evaluating, and optimizing medical imaging systems and mage processing and reconstruction methods. The two major components of simulation are: (1) a realistic model of the human anatomy and physiological functions, and (2) ability to generate accurate image data that include effects of the imaging process. Without such, the results of the simulation may not be indicative of what would occur in actual patients and would, therefore, have limited practical value. The current four- dimensional (4D) NURBS-based cardiac-torso (NCAT) phantom was developed to provide a realistic and flexible model of the human anatomy and physiology and is widely used in nuclear medicine imaging research. The phantom has the advantage, due to its design, that its organ shapes can be changed to ealistically model different anatomical variations and patient motion. Although capable of being far more realistic, the NCAT phantom was designed for low-resolution nuclear medicine imaging research, and lacks the anatomical detail to be applicable to high-resolution CT. At the same time, current phantoms used in CT lack sufficient realism in depicting the complex shapes of real human organs and the flexibility to model anatomical variations and normal physiologic motion, deficiencies that are becoming increasingly important with rapid advancements in these imaging technologies and in the development of new applications. We seek to fill this void by building upon the existing 4D NCAT phantom and other simulation tools developed in our laboratory. We hypothesize that the tools developed in this work will provide simulated CT image data that accurately mimic that obtained from actual patients (male and female) at different stages of development (adult and pediatric). As x-ray CT evolves into many new applications and gains wider use, the simulation tools developed in this work will have applications in a broad range of imaging research in developing image acquisition strategies, image processing and reconstruction methods, and image visualization and interpretation techniques. Also, the tools provide the necessary foundation to optimize clinical CT applications so as to obtain the highest possible image quality with the minimum possible radiation dose to the patient. Due to radiation concerns it is impractical to optimize the large number of imaging parameters available in modern CT systems in human patients in ways that are specific to clinical demands. It is equally impractical to perform optimizations in physical test objects that cannot realistically duplicate the conditions seen in vivo. Such a task can only be practically and efficiently performed using accurate and realistic computer simulation methods, which have not yet been developed. Our team of investigators and consultants has extensive expertise in developing digital phantoms, accurate models of the medical imaging process, and 3D image reconstruction techniques and methods that are well suited for this project.